Conventionally, liquid crystal devices have been widely used as a screen for word processors and computers, and recently have been pervasive as a screen for televisions. Most of the liquid crystal display devices employ a TN (Twisted Nematic) mode. However, the TN mode liquid crystal display device has problems that contrast decreases and tone characteristic inversion is likely to occur when viewed on an angle.
To improve a viewing angle characteristic in an angled direction, a liquid crystal display device employing a VA (Vertically Alignment) mode has come to receive attention in these years. For example, a VA mode liquid crystal display device is disclosed in U.S. Pat. No. 6,384,889 (patented on May 7, 2002; hereinafter referred to as conventional example) corresponding to Japanese Laid-Open Patent Application No. 2000-47251 (Tokukai 2000-47251; published on Feb. 18, 2000). A liquid crystal cell of the VA mode liquid crystal device is composed of vertical alignment layers and a nematic liquid crystal having a negative dielectric anisotropy.
In the VA mode liquid crystal display device, liquid crystal molecules are aligned in a vertical direction under no applied voltage. When linearly polarized light from a polarization plate enters the liquid crystal layer having the liquid crystal molecules in this state, the light leaves the liquid crystal layer as linearly polarized light without changing the state of polarization, because the liquid crystal layer has almost no birefringence anisotropy. The linearly polarized light is then absorbed by a polarization plate located on the other side of the liquid crystal layer. As a result, the liquid crystal display device can produce black display.
On the other hand, when a voltage is applied, the liquid crystal molecules in the liquid crystal layer are tilted according to the applied voltage. Here, when the liquid crystal molecules are radially aligned as shown in the conventional example for example, the aligned direction of the liquid crystal molecules continuously varies even within a picture element region.
Further, among these types of liquid crystal display devices, there has been a liquid crystal display device in which a chiral dopant is added to vary the alignment of the liquid crystal molecules in a spiral manner along the thickness direction of the liquid crystal layer, as in the normal twisted alignment. This reduces a dark field portion, thereby improving brightness of the liquid crystal device.
In the foregoing conventional example, (U.S. Pat. No. 6,384,889), as described in the first paragraph of EXAMPLE 7 (corresponding to paragraph [0039] in Tokukai 2000-47251) for example, a chiral dopant is added so that a chiral pitch of 18 [μm], which is about four times the cell thickness, is obtained, and a twist angle is set to be about 90 degrees under applied voltage. Under these conditions, a serious quenching pattern remains over a large area and with high intensity, resulting in decrease in transmission intensity and decrease in brightness.
Further, the liquid crystal display device disclosed in U.S. Patent Application Publication No. 0036740 (published on Mar. 28, 2002) includes a structure in which liquid crystal has a twisted structure for the stable alignment of the liquid crystal. However, this publication does not have a notion of eliminating the quenching pattern, or improving the transmittance by eliminating the quenching pattern. Further, the publication does not disclose optimizing the quenching pattern or transmittance.